In an attempt to elucidate the laminar/turbulent transition mechanism in a Blasius boundary‐layer flow, a nonsemisimple resonance of phase‐locked secondary instability modes is investigated. Thelocalnonlinear behavior is described by means of a center manifold reduction. The numerically computed normal form is of the symmetric Takens–Bogdanov type and predicts a homoclinic orbit which is possibly related to a physical bursting process. Aglobalcontinuation procedure for equilibrated three‐dimensional (3‐D) waves in the full Navier–Stokes system validates some of the local predictions and very closely outlines the experimentally observed skin friction domain including subcritical transition. ©1995 American Institute of Physics.